Optical characterization at the nanoscale currently requires instruments such as NSOM (near-field scanning optical microscope), TERS (tip-enhanced Raman spectroscopy), or hybrid AFM (that includes a specialized far-field optical microscope).
Attempts at integrating atomic force microscopy and optical techniques have already been made and several products based on these schemes have found their way into the market. Commercially available, AFM tips with integrated waveguides (hollow tips) are used in conjunction with an external laser source [Celebrano 2009]. This high-cost approach suffers from inherent limitations in terms of optical resolution and light power that can be delivered. To achieve high lateral resolution the size of the near-field aperture needs be reduced, thus leading to an exponential decrease of optical power output. This approach has limited applications to near-field microscopy with ultimate resolution of about 50 nm, but is not appropriate for optical spectroscopy because of small power output.
Other approaches aimed at better integration of light source and AFM tip have generally involved either attaching a prefabricated light source (edge emitter, VCSEL, or LED) above a Si AFM cantilever probe (hybrid approach) [Bargiel 2006, Kingsley 2008] or fabrication of the light source directly on the AFM tip [Heisig 2000a, Heisig 2000b, Hoshino 2008, Hoshino 2009]. In these instances, the optical detectors were not integrated into the probes. The hybrid approach has only been shown to work in research labs and it is difficult to imagine how optical probes can be fabricated cost-effectively to make them affordable for a wider scientific community. In addition, VCSELs, commonly used in this approach, are limited in their optical output power.
In addition, single, integrated photodetectors have also been fabricated on AFM tips [An 2008]. The photodetector-only approach [An 2008] does not address the difficulties of aligning the light source onto the AFM tip, and the requirement to reduce detector size in order to achieve spatial resolution directly contradicts the requirement to have the largest possible detection area necessary to obtain high sensitivity levels for optical spectroscopy on nanoscale.
Lastly, an AFM tip with integrated LED light source and photodetector has been demonstrated [Sasaki 2000], but while the photodetector was monolithically fabricated into the probe, the light source (a GaAs LED) was simply glued onto the cantilever chip. This is insufficient to meet the requirement of high power, single wavelength operation.